1. Field of the Present Invention
The present invention relates to a radio communication system, a radio communication apparatus and a radio communication method, and a computer program in which a plurality of mobile stations simultaneously communicate with a base station by means of multiple accesses, in particular to a radio communication system, a radio communication apparatus and a radio communication method, and a computer program in which time division multiple access (TDMA) communication is carried out for each base station cell.
More particularly, the present invention relates to a radio communication system, a radio communication apparatus and a radio communication method and a computer program in which a mobile station performs communication according to a TDMA frame synchronized with a control channel which is transmitted to each cell from a base station; still specifically, relates to a radio communication system,. a radio communication apparatus and a radio communication method, and a computer program in which under a multi-cell radio communication environment where connection between cells or TDMA frame synchronization is not established, a mobile station carries out an adjacent cell search with low power consumption in the event of a handoff and the like.
2. Description of the Related Art
Mobile communication has originally arisen from the discovery of electromagnetic waves and subsequent to the discovery, research and development have been carried out from a need for communications with vessels, aircraft, trains and the like. Further, objects to be communicated have been expanded to cars and persons as well. It has been possible to transmit not only transmission data by means of telegraphy and telephony but also computer data and multimedia content such as images.
Recently, due to improvements in manufacturing technologies a mobile terminal has rapidly become smaller and less costly. Also an expansion of information and communication services has contributed to a personalization of mobile terminals, such as the introduction of cellular phones. Further, deregulation and liberalization of communications and reduction in communication charges have increased the number of users.
In mobile telecommunications, basically a mobile station such as an in-vehicle phone and a cellular phone finds a nearest base station and electromagnetic waves are exchanged between the mobile station and the base station. A range of communication in which electromagnetic waves from a base station can reach is referred to as “Cell”. A cell is usually a circle having a predetermined radius and centered on a base station antenna. Arranging cells without a gap constitutes a communication service area.
In FIG. 11, there is schematically illustrated a cell arrangement of a mobile radio communication system, as exemplified by a cellular system, where a plurality of base stations are deployed to form a service area. The service area is a multi cell structure in which the base stations (not shown) are disposed in a predetermined space interval. As shown in the figure, the plurality of cells each provided by the base stations are prepared as a continuum (or so as not to be isolated) so that a mobile station may perform communication from anywhere in the service area, whereby an expanded service area is achieved.
The reason why the mobile communication system uses cells in this manner is that there are advantages in that by allowing electromagnetic waves of a base station to reach only within its cell, another cell may use the same frequency (again) and limited frequency resources may be effectively utilized; by dividing into cells, an output of electromagnetic waves for communication is reduced so that a mobile device worn as a portable device driven by batteries may be reduced in size and in electric power, for example. Recently, according to an increase in the number of cellular phone users, cells are becoming smaller and smaller in size.
As a matter of fact, there is a plurality of mobile terminals in a cell so that the mobile terminals communicate with a base station at the same time. In other words, from the base station's point of view, it is necessary to detect what signal corresponds to what user (multi-user detection) by multiple accesses or multiplexing radio signals.
As multiple access technologies, Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) may be quoted, for example.
The TDMA is a communication system in which communication channels are divided into time slots on a time base so as to allocate different time slots to different mobile terminals performing communications at the same time; a digital system is a presumption for this type of system.
On the other hand, FDMA is a communicating system in which different frequencies are allocated to mobile terminals communicating with at the same time (that is, a frequency is allocated to each communication channel). In other words, multiple channels used for communication are arranged on a frequency base so that channels that are free are appropriately allocated. The FDMA may be applied to either analog or digital communication systems.
For example, the base station may accommodate a plurality of mobile stations within the same cell at the same time by means of a TDMA system. In this case, a TDMA frame is divided into a plurality of time domains to be allocated to uplink and downlink of each mobile station. For allocating time domains, a method of resource reservation may be employed, for example.
Further, in the FDMA system frequency channels to be used may be switched between cells so as to perform communications, thus solving a problem of interference waves from outside the cells , which occur around cell boundaries.
Within the cell, the base station periodically transmits (broadcasts) control information such as a beacon so as to perform synchronization within the cell, identification of the cell, designation of a frequency channel to be used, and the like.
On the other hand, as shown in FIG. 11, in a service area where small-sized cells are arranged without a gap, as the mobile station moves from cell to cell a base station to be connected must be changed. Such a changeover between connection base stations carried out by the mobile station is referred to as “handover” or “handoff”. The smaller a cell becomes, the more often a handoff happens.
For each handoff, it is necessary for the mobile station to search with which base station to link next or to perform a cell search. For example, as the mobile station moves to a periphery of the cell, a communication quality of desired signal from the connected base station decreases (RSSI: a Received Signal Strength Indicator decreases) and interference signals from outside the cell may be received, thereby being necessary to search another base station of high communication quality. As a matter of course, the mobile station searches a cell when activating its device.
When the mobile station performs a cell search, a control channel such as a beacon (identification signal) transmitted by a base station of an adjacent cell should be received. In this case, switching a frequency channel to another frequency channel except for busy ones in the connected cell, the base station waits for a control channel of higher signal strength to be received.
For example, in a wide area cellular system, due to the presence of a backbone, a control channel may be transmitted at the same timing in all cells by cooperating among base stations and establishing synchronization. In other words, TDMA frames are synchronized between cells. In such a case, when the mobile station needs a cell search, it waits for a control channel at the same timing period as the previously connected base station so as to easily search a cell.